The invention relates to a method for the manufacture of a silver catalyst for electrochemical cells, particularly fuel cells, wherein metallic silver is precipitated on a metal hydroxide gel by reduction of a silver salt, and 0.5 to 5% by weight of hydroxides, referred to the silver, of the metals bismuth, nickel and titanium are utilized, as well as to a silver catalyst manufactured by this method.
It is known to use silver as a catalyst for the oxygen reduction in positive electrodes of electrochemical cells such as fuel cells. For example, in fuel cells with alkaline electrolytes, Raney silver has proven useful. The preparation of Raney silver presents difficulties, however, particularly in the milling of the melt regulus of silver and aluminum due to the ductility of the silver-aluminum alloy.
The difficulties arising in the preparation of Raney silver may be avoided by a method, known from Canadian Patent 977,735, for the manufacture of a silver catalyst for electrodes of electrochemical cells. In this method, metallic silver is precipitated on a gel-like metal hydroxide by reduction of a silver salt. Subsequently, the metal hydroxide is dissolved. The metal hydroxide may be aluminum hydroxide, cobalt hydroxide, cadmium hydroxide or nickel hydroxide and the gel-like metal hydroxide is first precipitated by the addition of a base to a metal salt solution. A reducing agent is then added to the suspension obtained and to this mixture is added a solution of the silver salt. The silver salt is reduced to metallic silver and the silver is precipitated on the metal hydroxide gel. Subsequently, the metal hydroxide is dissolved, at least partially, from the silver-metal hydroxide mixture.
To some extent, however, this method remains somewhat expensive, since several process steps are necessary. In addition, the major portion of the original carrier material, i.e., the metal hydroxide gel, is removed, which is economically undesirable. These disadvantages may be circumvented in a method known from U.S. Patent 3,900,342, by utilizing in the preparation of a silver catalyst (through precipitation of metallic silver on a metal hydroxide gel in the course of the reduction of a silver salt), 0.5 to 5% by weight of commingled hydroxides, referred to the silver, of at least two of the metals titanium, iron, cobalt, nickel and bismuth. In addition to overcoming the above-noted disadvantages, the catalyst obtained in this manner exhibits increased activity.
In the silver catalyst prepared in accordance with this known method, one of the metal hydroxides contained in the catalyst is preferably bismuth. Of other metal hydroxides, the following combinations, among others, may also be present: cobalt and nickel hydroxide, nickel and titanium hydroxide, or iron, nickel and titanium hydroxide.
Among these silver catalysts, a catalyst with bismuth, nickel and titanium hydroxide, which is referred to hereinafter as an Ag.sub.BiNiTi catalyst, has particularly been found to be highly suitable for the cathodic reaction of oxygen, specifically with respect to the activity and long-term behavior. While this catalyst is also suitable for the reaction of air oxygen, the long-term behavior is not quite satisfactory. This is disadvantageous since H.sub.2 /O.sub.2 fuel cells become energy generators of economic interest only if air instead of oxygen can be used as the operating gas, i.e., as the oxidant.